1. Field of the Invention
The present invention relates generally to a wireless communication system using Multiple Input Multiple Output (MIMO) technology (hereinafter referred to as a “MIMO wireless communication system”), and in particular, to a channel estimation method in a MIMO wireless communication system.
2. Description of the Related Art
To provide services having various qualities of services (QoSs) at around a 100 Mbps data rate in 4th generation (4G) communication systems, i.e., next generation communication systems, a large amount of research has been performed. In particular, research to support a high-speed data service with guaranteeing mobility and QoS in Broadband Wireless Access (BWA) communication systems, such as Local Area Network (LAN) systems and Metropolitan Area Network (MAN) systems, of the 4G communication systems has been conducted.
In the 4G communication systems, research on multi-antenna schemes has also been performed as an alternative plan to overcome limitation on an allocated bandwidth, i.e., to raise a data rate. The multi-antenna schemes can overcome limitation on frequency-domain bandwidth resources by utilizing a space domain.
For example, a mobile communication system is structured such that a plurality of user equipments communicate with each other via a base station. When the plurality of user equipments communicate with each other via the base station at a high data rate, a fading phenomenon occurs due to characteristics of wireless channels. To overcome the fading phenomenon, a transmission antenna diversity scheme, i.e., one of the multi-antenna schemes, has been suggested. The transmission antenna diversity scheme is used to increase a data rate by minimizing a data transmission loss due to the fading phenomenon by transmitting signals using at least two transmission antennas, i.e., multiple antennas. This transmission antenna diversity scheme will now be described herein below.
The transmission antenna diversity scheme is used to cope with distortion, which is caused by the fading phenomenon, of received transmission signals that have undergone independent fading phenomena in a wireless channel environment. Transmission antenna diversity scheme is classified into a plurality of schemes such as a time diversity scheme, a frequency diversity scheme, a multipath diversity scheme, and a space diversity scheme. That is, a mobile communication system must fully overcome the fading phenomenon, which most seriously affects communication performance, in order to perform high-speed data transmission because amplitude of a received signal is decreased by several dB to tens of dB because of the fading phenomenon.
The time diversity scheme is used to effectively deal with burst errors generated in a wireless channel environment using interleaving and coding technologies, and is generally used in a Doppler spread channel. However, the time diversity scheme has a drawback in that it is difficult to obtain a diversity effect in a low-speed Doppler channel. The space diversity scheme is commonly used in a channel with low delay spread, such as an indoor channel, and a low-speed Doppler channel with low delay spread, such as a pedestrian channel. The space diversity scheme is used to obtain a diversity gain by using at least two antennas. That is, when a signal transmitted from an antenna is attenuated because of a fading phenomenon, the space diversity scheme is used to obtain a diversity gain by receiving signals transmitted from the remaining antennas. The space diversity scheme is classified into a reception antenna diversity scheme utilizing a plurality of reception antennas, a transmission antenna diversity scheme utilizing a plurality of transmission antennas, and a MIMO scheme utilizing a plurality of reception antennas and a plurality of transmission antennas.
In the MIMO scheme, a data rate is increased by using a spatial multiplexing scheme and a space-time coding (STC) scheme. The spatial multiplexing scheme is used to multiplex an information data signal into as many parallel data streams as the number of transmission antennas, and then transmit the data streams through independent paths via the transmission antennas. A Bell Labs Layered Space Time (BLAST) scheme exists as the spatial multiplexing scheme. The use of the spatial multiplexing scheme can increase a data rate in proportion to the number of transmission antennas in a wireless channel environment without additional transmission power and frequency band.
In order to provide services having a variety of QoSs at around a 100 Mbps data rate to users, the Institute of Electrical and Electronics Engineers (IEEE) 802.11n system attempts to establish a next generation Wireless LAN (WLAN) standard. For the IEEE 802.11n system, a task group was organized on May 2003, and research to maintain compatibility with an IEEE 802.11a system, which is a conventional WLAN standard, and add the MIMO technology to the IEEE 802.11a system has been conducted. The IEEE 802.11a system uses Orthogonal Frequency Division Multiplexing (OFDM) technology to realize a data rate with up to a maximum of 54 Mbps. Therefore, the IEEE 802.11n system also can be realized by adding the MIMO technology to the OFDM technology. Hereinafter, a MIMO communication system based on the OFDM technology is called a “MIMO-OFDM system.”
The IEEE 802.11n system must perform channel estimation using a preamble used in the IEEE 802.11a system because compatibility between the IEEE 802.11n system and the IEEE 802.11a system must be maintained as described above.
FIG. 1 illustrates a frame of a conventional IEEE 802.11a system. Referring to FIG. 1, the frame can be divided into preamble fields 102 and 104, a signal field 106, and a data frame field (or Physical layer convergence protocol Service Data Unit (PSDU)) 108. The preamble fields 102 and 104 can be classified as a short preamble (SP) field 102 and a long preamble (LP) field 104. In general, the preamble fields 102 and 104 are used for channel estimation, sync acquisition and offset estimation between a user equipment and a base station.
More specifically, the SP field 102 is used for time synchronization and coarse frequency offset estimation, and the LP field 104 is used fine frequency offset estimation and channel estimation. Here, inter-preamble interference can be prevented by inserting a guard interval (GI) 103 between the SP field 102 and the LP field 104. The signal field 106 includes information indicating a length and a data rate of the subsequent PSDU 108.
A plurality of methods have been suggested for applying the preamble structure used in the IEEE 802.11a system illustrated in FIG. 1 to an IEEE 802.11n system. However, there are numerous drawbacks, which have not been solved, in these methods. The preamble design methods suggested considering an IEEE 802.11n system will now be described with reference to FIGS. 2 through 4.
It is noted, however, that although a MIMO-OFDM system having a predetermined number of transmission antennas (Tx Ants) and the same number of reception antennas (Rx Ants) will be described with reference to FIGS. 2 through 4, the description can be applied to any MIMO-OFDM system having at least two Tx/Rx Ants.
FIG. 2 illustrates a preamble structure using a repeating preamble pattern suggested for a conventional IEEE 802.11n system. Referring to FIG. 2, a signal having the same pattern as the preamble structure of the preamble fields 102 and 104 and the signal field 106 illustrated in FIG. 1 is transmitted from a first Tx Ant of a 3×3 MIMO-OFDM system for a frame duration 202. However, unlike the frame structure of FIG. 1, a second signal field 204 follows the frame duration 202. The second signal field 204 includes information for enabling a receiver to identify whether the 3×3 MIMO-OFDM system uses the MIMO technology or Single Input Single Output (SISO) technology. An LP including a GI is transmitted from a second Tx Ant to a receiver for a frame duration 206. Accordingly, the LP is transmitted from a third Tx Ant to the receiver for a frame duration 208. Data is transmitted from the first, second, and third Tx Ants to the receiver after the frame duration 208.
As described above, a MIMO-OFDM system having a repeating LP transmission structure is efficient in terms of performance. However, the MIMO-OFDM system needs to transmit as many LPs as the number of Tx Ants and the transmission of the LPs accompanies overhead. That is, an increase in number of antennas linearly increases the overhead.
FIG. 3 illustrates a preamble structure using the diagonal loaded preamble pattern suggested for a conventional IEEE 802.11n system. Referring to FIG. 3, the same preamble is transmitted from first, second, and third Tx Ants of a 3×3 MIMO-OFDM system. Therefore, overhead required to transmit an LP for each Tx Ant does not exist. However, because the LP is simultaneously transmitted from the three Tx Ants, pilots transmitted from the three Tx Ants must be identified to perform channel estimation for each Tx Ant. Additionally, pilot spacing must be separated according to the number of Tx Ants.
For example, if the number of Tx Ants is 4, a pilot signal transmitted from a first Tx Ant must be transmitted at time T0, T4, T8, . . . , a pilot signal transmitted from a second Tx Ant must be transmitted at time T1, T5, T9, . . . , a pilot signal transmitted from a third Tx Ant must be transmitted at time T2, T6, T10, . . . , and a pilot signal transmitted from a fourth Tx Ant must be transmitted at time T3, T7, T11, . . . Therefore, a receiver performs channel estimation by performing interpolations according to the pilot spacing. Accordingly, an increase in number of Tx Ants increases an interpolation frame duration, causing deterioration in channel estimation performance.
FIG. 4 illustrates a Hadamard preamble structure suggested for a conventional IEEE 802.11n system. More specifically, FIG. 4 illustrates a Hadamard preamble structure suggested for a conventional IEEE 802.11n system.
Referring to FIG. 4, a portion of a frame transmitted from a first Tx Ant of a 4×4 MIMO-OFDM system for a frame duration 402 is configured by adding the same second signal field as the second signal field 204 of FIG. 2 to the preambles 102, 104, and 106, before the PSDU 108 of the IEEE 802.11a system of FIG. 1. LPs transmitted from first, second, third, and fourth Tx Ants for a frame duration 404 are configured to be orthogonal to each other as illustrated in FIG. 4, and data frames are transmitted from the first, second, third and fourth Tx Ants to a receiver. Here, in the frame duration 404, 4 LPs are configured per Tx Ant. Because the number of Tx Ants is 4, 4 LPs are configured per Tx Ant to perform channel estimation according to Tx Ant. However, if the number of Tx Ants is 5, the number of LPs for each Tx Ant in the frame duration 404 is 5. Therefore, in a MIMO-OFDM system using the Hadamard method, the number of additional preambles is increased along with an increase in number of Tx Ants.
As described above, an IEEE 802.11n communication system is used in a MIMO-OFDM communication system in which the MIMO technology is added to the OFDM technology. Also, the IEEE 802.11n communication system uses preambles of conventional IEEE 802.11 communication systems. Further, the IEEE 802.11n communication system can maintain compatibility with the IEEE 802.11 communication systems by using the same preambles. Accordingly, preamble structures suggested for the IEEE 802.11n communication system are slightly modified while maintaining a preamble structure of the IEEE 802.11a standard.
In the suggested preamble structures, an increase in channel estimation performance increases overhead. On the contrary, a decrease in overhead decreases the channel estimation performance. Therefore, in order to solve the problems of the conventional preamble structures, a new preamble structure is needed, which is capable of improving its channel estimation performance and reducing overhead.